Thyroid problems are common in pets. Several glands within the body produce hormones that have various functions, including controlling metabolism.  One of these is the thyroid gland, which produces thyroid hormone.

Not producing enough thyroid hormone, a condition known as hypothyroidism, is common in pets.  In hypothyroidism, the metabolism slows down.  Your pet may seem less active or listless, and it may seek out warm places.  Many hypothyroid pets tend to gain weight easily, and they often develop skin problems, including hair loss or thinning that is symmetrical (ie, the same on both sides of the body), dandruff, or darkening of the skin.

Hypothyroidism is most common in middle-aged and older dogs, especially medium to large breeds.  It can be caused by problems with either the thyroid gland itself or the pituitary gland.  The pituitary gland produces another hormone that regulates the release of thyroid hormone.  When there is a problem with the pituitary gland (eg, a tumor), it might not produce enough of the hormone that tells the thyroid gland to release thyroid hormone.  Another common cause of hypothyroidism is autoimmunity, which means that the immune system becomes confused and attacks the thyroid gland.

Sometime the thyroid gland produces too much hormone, a condition known as hyperthyroidism.  This problem is especially common in middle-aged to older cats, usually because of a tumor of the thyroid gland.  Signs of hyperthyroidism include weight loss, increased appetite (sometimes ravenous), and increased activity.  Sometimes, a noticeable lump, which is the enlarged thyroid gland, can be felt on the underside of the neck.

Your veterinarian can diagnose thyroid problems by measuring the level of thyroid hormone in the blood.  Hypothyroidism is treated by giving a daily oral medication to replace the missing hormone.  Usually, several weeks of treatment with the replacement hormone are needed to see substantial improvement.  Blood tests to measure the level of thyroid hormone are usually repeated every 6-12 months to make sure your pet is responding to the medication and to determine if the dosage needs to be adjusted.  Hyperthyroidism can be treated with either surgery to remove the thyroid gland, or medication that destroys some of the gland so that less thyroid hormone is produced.